1. Field of Invention
The present invention relates to a control system and method for controlling switching between 2-stroke and 4-stroke modes of Homogeneous Charge Compression Ignition (HCCI) engine.
2. Description of Related Art
Homogeneous Charge Compression Ignition (HCCI) technology provides an improvement in efficiency and significant reduction in emissions in comparison to Spark Ignition (SI) and Compression Ignition (CI) internal combustion technologies. The improved fuel economy of the HCCI engines is explained by a high dilution of charge and low throttling losses, which simultaneously results in lower Carbon Monoxide (CO) emissions. In addition, the HCCI engine achieves reduction of Nitrous Oxides (NOx) emissions by about two orders of magnitude in comparison to conventional SI and CI diesel engines, which reduction results from much lower combustion temperatures due to the high dilution of fresh charge.
One of the main challenges in implementing the HCCI regime is an initiation of the homogeneous combustion. Unlike the SI or CI engines in which combustion is initiated with spark or fuel injection, the HCCI engine lacks a specific event initiating combustion. The auto ignition of a premixed charge in the cylinder is achieved by controlling concentrations of the mixture components and their temperature at the beginning of a combustion stroke. Two main approaches for control of the mixture temperature are a heating or precompressing of an intake charge and reinduction or trapping exhaust gasses from the previous cycle. The latter approach results in higher overall efficiency and is implemented using active valve control system regulating valve timing.
The high dilution of a fresh charge with exhaust gasses in the engine cylinder is required to increase temperature at the beginning of the combustion stroke. However, the dilution results in a limited work output of the HCCI engine in comparison to SI and CI engines. Several methods are introduced to overcome this limitation. One of the methods employs a hybrid switching strategy which involves switching between HCCI and standard SI modes. When the load is low, the engine can operate in a highly efficient HCCI mode; when a higher output is required, the engine switches to the SI mode, though this means the advantages of the HCCI are lost. Another alternative, which allows the engine to operate in the broad range of loads, is a hybrid strategy involving switching between 4-stroke and 2-stroke HCCI modes. This method allows harvesting the efficiency and emission advantages of the HCCI technology without limiting of the load range. In addition, switching between the two HCCI modes is potentially simpler to implement than switching between 4-stroke HCCI and SI modes. The main idea of this approach is to use the 4-stroke HCCI mode at low loads and switch to the 2-stroke mode at high loads to double the number of combustion events per engine revolution while preserving all the advantages of the HCCI technology.